Autonomic nervous system (ANS) is able to change both heart beat-to-beat interval and peripheral muscle vascular tone in response to different stimuli. Unfortunately the direct measure of the sympathetic and vagal activity appears not feasible in a clinical setting. ANS modulation is studied non-invasively by means of heart rate variability (HRV) and baroreflex sensitivity. Decreased HRV has been found in critical ill patients with multiple organ dysfunction syndrome (MODS) and sepsis, thus it has been supposed being a sign of autonomic dysfunction. Frequently, in mechanically ventilated critical ill patients the HRV does not show any oscillatory pattern, as well as it appears in the early months after heart transplantation. Under these circumstances the heart seems to lack the neuro-modulatory control by ANS and it seems to respond exclusively to the preload and afterload laws. This could have implications for outcome because autonomic dysfunction is associated with increasing severity of illness and mortality. Since the ANS modulation is a dynamic process that implies a central integration of a complex variety of afferent stimuli (from carotid sinus, cardiopulmonary receptors, pain,…) and efferences through sympathetic and vagal branches, up to the present it is unclear if in critically ill a reduced HRV at rest reflects a state of low requirement of ANS modulation or truly a failure of the ANS. To provide new insights into this important topic we study the changes of ANS modulation in response to a orthostatic sympathetic stimulus daily from the day of ICU admission until day 28, or the day of discharge from ICU if it occurs before the day 28.